How to make a transition between 2 strips in NLA Editor?
Actually you can't add a transition to fill a gap.

If you want to interpolate 2 strips they have to overlap, therefore you have to change the speed of your action or add more "action" to your strips.

Speed


Select the strip
In the Properties Panel (P) > Action Clip, increase the Scale value of Playback Settings


Extrapolation

To extend a strip, you could set the channel extrapolation mode to Linear Extrapolation (or some loop) first, then increase the End Frame in the Action Clip Panel.

EDIT
Using this method, however, the last frame will be determinate by the generated curve and it could produce odd results (note the rotation of the cube at the end of the animation).

If you cannot scale the strip or re-arrange the strips, I suggest you to "fill the gap" editing (or adding) the actions keyframes